Transparent glass and glass-ceramic microspheres (i.e., beads) are used as optical elements for retroreflective signage, apparel, and pavement markings. Such microspheres can be produced, for example, by melting methods. Such melting methods may include melting a raw material mixture in the form of particulate material. The melted particles can be quenched, in air or water for example, to give solid beads. Optionally, quenched particles can be crushed to form particles of a smaller desired size for the final beads. The crushed particles can be passed through a flame having a temperature sufficient to melt and spheroidize them. For many raw material compositions this is a temperature of about 1500° C. to about 3000° C. Alternatively, the melted raw material composition can be poured continuously into a jet of high velocity air. Molten droplets are formed as the jet impinges on the liquid stream. The velocity of the air and the viscosity of the melt are adjusted to control the size of the droplets. The molten droplets are rapidly quenched, in air or water for example, to give solid beads. Beads formed by such melting methods are normally composed of a vitreous material that is essentially completely amorphous (i.e., noncrystalline), and hence, the beads are often referred to as “vitreous,” “amorphous,” or simply “glass” beads or microspheres.
Pavement markings including microspheres prepared from compositions that comprise lanthanum oxide and titanium oxide are described for example in U.S. Pat. No. 3,946,130 (Tung) and WO 96/33139.